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Process Gas Control System

The MAT-VAC Technology, Inc. series of sputtering systems all have a common process gas system. The core of which is the MKS-250 Gas Controller and the MKS-247 Four Channel Readout. The gas system can control up to four gas channels. There are several major components in the process gas system:

Process Gas Inlet Port Panel - Located rear of chassis and permanently plumbed
Mass Flow Controller (1 per gas channel) - Normally Installed on valve manifold panel
Pneumatic Gas Isolation Valve (1 per gas channel) - Normally attached directly to associated MFC
Electro-pneumatic Valve Bank (1 per gas channel) - Located on 100 PSI CDA Manifold adjacent to P60 Panel
Process Chamber Gas Distribution - As required, varies per customer specification
MKS-627 Baratron Gas Pressure Sensor - Mounted to chamber on 2¾" conflat fitting
MKS-250 Gas Controller - Chassis Panel Mounted
MKS-247 Four Channel Readout - Chassis Panel Mounted
PLC I/O - GASIN AI & GASOUT AO from CIOP, Gas Isolation Valve Wiring located on P60 panel

Refer to the block diagram above for the following discussion. The recipe selects the gas(es) and determines the gas setpoint. This data is passed down from the VB to the PLC where the PLC controls it's various outputs to open the Gas Isolation Valve(s) and command to the MKS-250 Gas Controller.

The MKS-250, in conjunction with the MKS-247 and the Mass Flow Controllers (MFC) and the MKS-627 Baratron sensor forms a closed loop control system where the MFC's flow rates are adjusted so that the MKS-627 Baratron read gas pressure is equal to the command setpoint. The output from the MKS-627 Baratron sensor is buffered and scaled and fed back to the PLC for eventual display on the VB status screens. Along the way the PLC compares the setpoint and command levels and derives the GASNEAR and GASOOS status variables that tell the VB if the gas system is stable enough to begin sputter.

The throttle valve is use to set the maximum gas pressure by limiting the chamber cryo pump. The gas should be manually set to the maximum desired pressure and the throttle position adjusted to allow the pressure to occur. Closing the throttle valve will increase the pressure, opening it will decrease the pressure. Once the throttle has been set, the set screw should be locked in place using the supplied jam nut.

The gas system is held off until the system is ready to sputter. Simultaneously with the sputtering power supply (DC or RF) being commanded on, the isolation valves between the MFC's and the chamber are opened. There is a small amount of high pressure gas in the infeed to the isolation valve that "bursts" into the chamber, momentarily raising the gas pressure. This "Gas Burst" helps insure plasma ignition even with RF sources. The PLC monitors the signal from the MKS-250 controller and waits for the gas pressure to stabilized before starting to scan.

As the gas begins to flow the Baratron Gauge senses the gas pressure inside the chamber itself. This pressure signal is sent to the Controller and it is compared to the gas setpoint as commanded by the PLC/VB. The error signal is used to drive the Mass Flow Controller(s) to meter the gas being sent to the chamber. It only takes a few seconds for the gas system to converge on the setpoint and it should remain there for the duration of the sputtering run.

At the end of the run the gas isolation valves are closed to completely stop the flow of gas into the process chamber. While you could theoretically command the MFC's to "zero flow" in practice they will always have a certain amount of seepage even with a zero flow command. This is why the isolation valves are used to insure that "zero flow" is actually "no flow". This both conserves process gasses (which tend to be expensive, and insures that the cryo can more easily acheive high vacuum levels between process steps.

MKS-250

The MKS-250 Gas controller is illustrated at right with the controls in their correct position. The controls on the front panel of the MKS-250 include:

Power Switch - On - Green PWR LED will illuminate
Internal Setpoint Knob - N/A
Internal/External Reference Switch - EXT
Input Range Switch - 1V
Input Zero Adjustment - Calibrate so LED reads 0.00 when chamber pressure is below 10^-5 Torr
Input Span Adjustment - Factory Preset - Do NOT adjust
Phase Lead Control - 1
Gain Control - 10
Bias Adjustment - Factory Preset - Do NOT adjust
Mode Selector - AUTO
Manual Setpoint Knob - N/A
LED Display

MKS-247

The MKS-247 Gas controller is illustrated at right with the controls in their correct position. The controls on the front panel of the MKS-247 include:

Power Switch - ON
1 - ON, Green LED above SW will illuminate
2 - ON if installed, OFF if not
3 - ON if installed, OFF if not
4 - ON if installed, OFF if not
Flow/Ratio/Ext - All to RATIO position
Display Channel - 1
LED Display